I have explained before on how to connecting the SPDIF input to the digital receiver and then goes to the digital filter. The IC that I use are YM3623 and CXD1144 here and here. You might wondering what would be my candidate DAC IC for this another ultimate DAC build, and here it is, the AD1865 DAC from Analog Device. The reason for this is because I still have several AD1865 in my bin. Other than that, especially for the sound quality, in my opinion AD1865 is sound great. It is R2R DAC and very simple to build compares to more complicated and rare PCM63 or AD1862.
The connection between CXD1144 to AD1865 is explain in the AD1865 datasheet but mentioned as CXD1244. The difference between CXD1144 and CXD1244 are on pins arrangement, while all the correspondences pins are basically the same. One should be mentioned that CXD1244 are not the successor of CXD1144. CXD1244 is the lower grade digital filter as shown in the datasheet specification.
Before I go through to the connection between CXD1144 and AD1865, in the photo bellow, I mentioned about the voltage requirement for this AD1865. They are +5v, -5v for analog voltage input and another +5v for digital voltage input. The digital +5v voltage input can be tie up with the CXD1144 +5v as well. For better audio performance the voltage between analog and digital should be separate each others. This can be done by using individual voltage regulator.
In the photo bellow, I add the AD1865 DAC schematic continuing from my last drawing.
The connection for the digital input only require 4 connection. They are BCK, WS, DR and DL. All pins explain in the schematic bellow.
I will using the current output from pin 4 and 21 on each channel in AD1865. The reason for using external IV converter is because I can using higher voltage input for the op-amp IC. In this case I using +9v and -9v compares to internal AD1865 using the same voltage as the analog voltage input. Again theoretically it would be better.
You can using the exact schematic recommendation on AD1865 datasheet using internal IV converter then going to LPF section. I have tried it as described in the AD1865 datasheet here. It working good and sound great.
The schematic for the IV converter using op-amps is very common. I draw the schematic as the photo bellow.
The schematic is for one channel. The op-amps that I using here is 5532. The cheap but very good sounding op-amps.
The capacitor bypass that I am using across the IV resistor is the 1st order LPF. You can trial it with several value from 220pF up to 2,2nF. The value will determine the LPF high frequency cut-off. You can browsing online to get this LPF calculator about frequency cut-off with different value capacitor of it.
Because the current output from AD1865 is 1 mA, then the resistor across the op-amps will determine on how high is the signal voltage output. As the formula 1 k ohm will convert 1 mA to 1 volt, then as per schematic above, the 2k2 will convert to 2,2 volt output. Easy right!
The next section after IV converter is the LPF section. I saw from many DAC build, this LFP get less attention. The worse is when using non-oversampling method without proper LPF design. The sound could be awful because of lacking anti alias filter from the digital conversion by the DAC.
For easy LPF schematic, again you can follow the AD1865 datasheet on page 8. The datasheet recommendation is using Butterworth LPF design. But in this project, I am using the Sallen-key LPF design with cutting high frequency at 48 kHz. Again, you can check the calculator first by searching online.
That's all for the schematic. It is all done.
The next post will be the final part about assembly all components together.
Disclaimer: Any statement and photos in this article are not allowed to copy or publish without written permission from the writer. Any injury or loss from following tips in this article is not under writer responsibility.
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